1. Field of the Invention
This invention relates to a laser beam machining method and a laser beam machine for subjecting workpieces to machining using a laser beam.
2. Description of the Related Art
Laser beam machines are used for obtaining products of desired shape by cutting plate-like workpieces using a laser beam. On the occasion of the cutting work, piercing work is performed first. With the through-hole formed by piercing as a starting point, a machining head is moved to cut in accordance with a necessary locus such as a product's shape whereby the workpiece is cut into the desired shape.
In order to reduce the piercing time and to prevent the protrusion of molten substance on the surface of the workpiece, there have heretofore been proposed various methods.
The following methods (1)-(3), for example, have been proposed so as to do piercing work in a short time:
(1) As disclosed in Japanese Patent Unexamined Publication No. Hei.7-9175, while a nozzle is kept apart from the surface of a workpiece when piercing work is started, the workpiece is irradiated with a laser beam. As the piercing work progresses, the nozzle is gradually moved close to the workpiece.
(2) As disclosed in Japanese Patent Unexamined Publication No. Hei.5-138382, while the duty ratio of pulse oscillation is set lower when piercing work is started, and an laser output is raised by gradually raising the duty ratio of the pulse oscillation as the piercing work progresses.
(3) As disclosed in Japanese Patent Unexamined Publication No. Hei.5-111784, a nozzle is moved toward and away from a workpiece periodically when piercing work is performed.
However, these methods (1)-(3) are intended to decrease the molten substance left on the surface of the workpiece as much as possible when the piercing work is performed and further, by the use of the assist gas blown from the nozzle, to blow off the molten substance as well as oxidation. Therefore, a great deal of machining time has been needed because the piercing work cannot be performed in a short time, although the protrusion of the molten substance is prevented to some degree.
Further, the following methods (4)-(7) have been proposed as those for preventing molten substance from blowing up during cutting work:
(4) As disclosed in Japanese Patent Unexamined Publication No. Hei.5-84589, high-purity oxygen is used as the assist gas blown out of a center nozzle to the machining portion of a workpiece when cutting work is performed. And, a double structure including the center nozzle and a shielding nozzle is employed so as not to drag the air in the machining portion but to lower the flow rate. Thus, it has been arranged that shielding gas is blown out of the shielding nozzle around the assist gas blown out of the center nozzle.
(5) As disclosed in Japanese Patent Unexamined Publication No. Hei.9-216081, oxygen pressure from an auxiliary nozzle is set high so as to blow off molten metal.
(6) As disclosed in Japanese Patent Unexamined Publication No. Hei.6-198485, a double-structure nozzle is employed and a cooling gas is jetted from around a shielding gas, so as to reduce cutting width.
However, each of these methods (4)-(6) is intended to do ordinary cutting work satisfactorily by letting the nozzle have a double structure. Moreover, high-pressure oxygen and a cooled inert gas are used as auxiliary assist gases, so that these gases are continuously blown out during the cutting work for the purposes of blowing off the molten metal and suppressing oxidation. Therefore, when the cutting of a corner portion is started, these methods cannot effectively prevent molten substancefromblowingup inthatcorner portion. Inother words, excessive calorie and oxygen per unit hour causes a large amount of molten substance to be produced since the cutting speed in the corner portion is low. Further, the purity of the oxygen is lowered in the case of continuously blowing out the inert gas against the workpiece, so that oxidative action of material to be cut is damaged, and thus, good cutting operation is not obtained.
There is another method (7) proposed for decreasing the protrusion of molten substance around a through-hole when piercing work is performed.
(7) As disclosed in Japanese Patent Unexamined Publication No. Hei.9-277071, piercing time is reduced by simplifying the operation of a nozzle when the piercing work is performed in a workpiece. In addition, in order to overcome a conventional troublesomeness which moves the nozzle upwardly so that a gap between the nozzle and the protrusion of the molten substance formed in the workpiece is secured when the cutting work is performed and to remove the molten substance securely, this publication discloses that a blow gas is blown from a blow nozzle while blowing an assist gas from a center nozzle, so as to blow off the molten substance.
In the method of (7), the blowing of the blow gas is started after the passage of a predetermined time after piercing work is started so as to prevent the assist gas from being cut off, and then the blow gas is continuously blown out until the piercing work is completed, so that the protrusion of the molten substance is removed. However, the continuous blowing of the blow gas after the passage of the predetermined time keep interrupting the flow of assist gas, and thus it has made ineffective to reduce the machining time because oxidative reaction according to the assist gas is suppressed. Further, since, in the piercing work, only the assist gas blows off the molten substance before the blowing of the blow gas, it has also made ineffective to reduce the piercing time.